Controlling Pressure Surges - Part 1
Pressure surges are generated by anything that causes the liquid velocity in a line to change quickly. The most common sources of pressure surge are rapid closing valves, pump start up and pump shut down. A pressure surge can consist of multiple events, resulting in surge pressures up to 10 times the normal pipeline pressure. These surges can be damaging to equipment resulting in lost profit and potentially loss lives.
The United States Department of Transportation (DOT) mandated guidelines, DOT Title 49 CFR Chapter 1 states “No operator may permit the pressure in a pipeline during surges or other variations from normal operations to exceed 110% of the operating pressure limit established under paragraph (a) of this section. Each operator must provide adequate controls and protective equipment to control pressure within this limit”.
All facilities such as pipelines and storage terminals should be protected by some form of pressure surge relief.
The two most common methods for pressure surge protection are surge relief valves and surge tanks. This article will explain the operation of both of these methods and will include the advantages and disadvantages of each.
Surge Relief Valves
Commonly used valves for surge protection are gas loaded, spring compression, pilot operated, and rupture pin. In this article it would be impossible to do justice to all the different types of valves. We will therefore focus on the operation and advantages of gas loaded piston type valves.
These valves utilize a pressurized volume of gas on top of a valve piston to create an actuation pressure (referred to as set pressure). This gas pressure should be monitored and regulated to maintain an accurate pressure relief set point.
Advantages:
1. Fast speed of response with soft closure to prevent generating a second surge event.
2. Gas loaded valves can be used on high viscosity products. Pilot operated valves can be limited by viscous fluids plugging the pilots.
3. Good flow characteristics (Cv).
4. Re-seats at the set point.
Disadvantages:
1. Repeatability is dependent on the design of the set point pressure controller.
2. Requires a discharge tank for pressure surge.
Bladder Type Surge Tanks
Surge relief tanks equipped with bladders made from highly elastic rubber can be used to reduce pressure surges
The bladder will typically be pre-charged at installation with a pre-determined pressure to give the correct operating level in the tank when the unit is connected to the main and at the operating pressure.
Advantages:
1. Very fast response time.
2. Zero loss of product because no product is diverted to a tank.
3. Acts as a suppressor of pulsating flow as well as surge relief.
Disadvantages:
1. Limited capacity for surge relief.
2. Surge suppressor must be as physically close as possible to the area where the surge is generated.
3. Limited maximum working pressure.